1. Field of the Invention
The present invention relates to an anode material for lithium secondary battery which has a large capacity, high safety and excellent charging and discharging cycle property; a lithium secondary battery using the above anode material; and a method for charging of the secondary battery.
2. Description of the Related Art
As electronic appliances have become smaller and lighter, the batteries used therein are required to have a higher energy density. There are also required secondary batteries allowing repeated charging and discharging, from the standpoint of resource saving. In order to respond to these requirements, secondary batteries using lithium have been proposed and developed. At the initial stage of development of secondary batteries using lithium, metallic lithium was used as the anode material. The secondary batteries using metallic lithium, i.e. metallic lithium secondary batteries, however, are inferior in rapid chargeability and have a short cycle life; moreover, they form dendrite at the anode (this may cause firing and explosion) and have a problem in safety. In order to solve the problem, there are currently in wide practical use lithium secondary batteries using, in the anode, a carbon-based material and/or a graphite-based material, i.e. lithium ion secondary batteries.
In order to allow various lithium secondary batteries including lithium ion secondary batteries to have a higher capacity, researches are being continued on the cathode material, anode material and electrolyte used therein. As the cathode material, LiCoO2 has been used mainly. The reason is that the cathode using this material is easy to produce and has relatively high safety. Lately, it is being investigated to produce a cathode using LiNiO2 or LiMn2O4 both having a larger theoretical capacity than LiCoO2 has.
With respect to the anode material, metallic lithium has a far larger theoretical discharging capacity (4,000 mAh/g) than graphite (372 mAh/g) but has problems in short cycle life and safety. Therefore, in order to solve these problems and achieve a higher capacity, active researches are under way still on a secondary battery using metallic lithium as the anode material. Researches are under way also on a lithium secondary battery using, as the anode material, a lithium alloy having a discharging capacity close to that of metallic lithium.
Various investigations have been made as well on the electrolyte of lithium secondary battery. Researches have been made also on the improvement of solid electrolyte in lithium solid secondary battery or on the improvement of polymer electrolyte in polymer lithium secondary battery.
It is not an exaggeration to say that the high capacity to be possessed by a lithium secondary battery depends upon the capacity level of the anode material used therein.
With respect to the electrodes of lithium secondary battery, there are reports by A. M. Wilsons et al. In J. Appl. Phys. 77(6), Mar. 15, 1995 and The Electrochemical Society Proceedings Volume 94-28, it is disclosed to synthesize a silicon compound material wherein silicon particles having particle diameters of nm order are dispersed in carbon, by chemical vapor deposition, i.e. by heating the vapor of a silicon-containing compound and the vapor of a carbon-containing compound in a quartz tube to give rise to a reaction and depositing the reaction product. It is also disclosed that each component is very finely dispersed in the above silicon compound material and therefore the material is substantially silicon carbide and that an anode made using the silicon compound material has a discharging capacity of about 500 mAh/g and good cycle property. This silicon compound material, however, is difficult to synthesize on an industrial scale.
As described above, in the researches of lithium secondary battery, it has been attempted to use a lithium alloy as the anode material. As the lithium alloy, there can be mentioned, for example, lithium-tin alloy, lithium-lead alloy, lithium-bismuth alloy, lithium-aluminum alloy, lithium-arsenic alloy, lithium-silicon alloy and lithium-antimony alloy. These alloys can be used per se as an anode material. However, in many cases, a metal or semimetal capable of forming a lithium alloy is used as an anode material and a battery is assembled. Upon charging of the battery, the metal or semimetal is electrochemically reacted with the lithium liberated form the cathode of the battery, to form an alloy, and this alloy is used as the anode material of the battery. In this battery, however, the volume of the anode expands several-fold at the time of alloying as compared with the volume before alloying; therefore, the alloy is powderized inevitably. Thus, the battery has no improvement in safety or cycle property. Therefore, no lithium secondary battery using a lithium alloy as the anode material is in practical application.
In order to realize an excellent lithium secondary battery, the present inventors made an intensive study for development of an anode material composed of a lithium alloy free from the above problem. The biggest problem in using a lithium alloy as the anode material of lithium secondary battery lies in that volume expansion takes place in formation of a lithium alloy and consequently the powderization and destruction of anode arises.
In order to solve the problem of electrode powderization and destruction, the present inventors made a further study. As a result, the present inventors found out that the problem of electrode (anode) powderization and destruction can be prevented by covering a metal or semimetal capable of forming a lithium alloy, with carbon.
The present inventors further found out that when a metal or semimetal capable of forming a lithium alloy is covered with carbon, the resulting carbon layer has an inhibitory action against the expansion of the metal or semimetal capable of forming a lithium alloy and, as a result, the powderization and destruction of electrode (anode) can be prevented.
That is, by using the particles of a metal or semimetal capable of forming a lithium alloy, as a core and covering this particulate core with carbon, a composite material having a double structure of a particulate core and a carbon layer can be obtained. In this composite material, the carbon layer has an inhibitory action against the expansion of the metal or semimetal capable of forming a lithium alloy. When this composite material is charged, the particulate core, i.e. the metal or semimetal becomes a lithium alloy. In this case, however, since the expansion of the metal or semimetal during its alloying is suppressed by the strong inhibitory action of the carbon layer, the powderization and destruction of electrode (anode) is prevented.
Next, it was made clear that as the metal or semimetal constituting the particulate core, capable of forming a lithium alloy, there is preferred titanium, iron, boron, silicon or the like, and silicon is particularly preferred.
It was also made clear that, of various methods for covering the particulate core with carbon, chemical vapor deposition is particularly preferred. By using this chemical vapor deposition, a strong inhibitory action against the volume expansion of the particulate core is obtained and uniform and complete covering is made possible with a small amount of carbon.
The present inventors made a study also on the charging conditions to be employed when the composite material obtained as above is used as an anode material for lithium secondary battery. As a result, there was obtained a lithium secondary battery which has high safety and high capacity, which is free from the problems of the prior art, and which has excellent properties not seen in conventional lithium secondary batteries. The present invention has been completed based on the above findings.
The anode material and secondary battery according to the present invention can be easily produced on an industrial scale.
An object of the present invention is to provide an anode material which is free from the above-mentioned problems, which causes neither powderization nor destruction of electrode, and which can realize a lithium secondary battery high in discharging capacity and safety and superior in cycle property. Further objects of the present invention are to provide a lithium secondary battery using the above anode material, and a method for charging of the secondary battery.
The present invention are as described below.
1. An anode material for lithium secondary battery, comprising
a particulate core composed of a metal or semimetal capable of forming a lithium alloy, and
a carbon layer covering the surface of the particulate core.
2. An anode material for lithium secondary battery, comprising
a particulate core composed of silicon, and
a carbon layer covering the surface of the particulate core.
3. An anode material for lithium secondary battery according to the above 1 or 2, wherein the average particle diameter of the particulate core is 0.1 to 50 xcexcm and the specific surface area of the anode material is 5 m2/g or less.
4. An anode material for lithium secondary battery according to the above 1 or 2, wherein the carbon content of the anode material is 5 to 50% by weight.
5. An anode material for lithium secondary battery according to the above 1 or 2, wherein the carbon layer covering the surface of the particulate core is formed by chemical vapor deposition.
6. An anode material for lithium secondary battery according to the above 1 or 2, wherein the carbon in the anode material has a lattice constant C0(002) of 0.680 to 0.720 nm.
7. A lithium secondary battery using, in the anode, an anode material set forth in the above 1.
8. A lithium secondary battery using, in the anode, an anode material set forth in the above 2.
9. A method for charging of a lithium secondary battery set forth in the above 8, wherein the charging density is 1,500 mAh/g or less.
10. A method for charging of a lithium secondary battery set forth in the above 8, wherein the final charging voltage is 30 to 100 mV (reference electrode=metallic lithium).